Southern Pygmy Mice are tiny, yet comparatively large for the genusBaiomys. They have short, smooth fur, short whiskers, and big shell-shaped ears. When they are born, their coats are dusky-gray. As they mature they molt twice, and by about two months of age have adult coats, which can vary from reddish-brown through almost black. Hot, dry, grassy fields where rocks or cacti provide protection make good habitat. There they can be fairly common, scampering along tiny runways that are often littered with green droppings. They burrow under dense clumps of grass or shelter under rocks. They eat nuts, bark, seeds, grasses, and leaves, and are active by day and into the early evening. They can apparently breed all year, but few pregnant females have been found in winter and spring. They have litters of 1-4. There are fossils of extinct pygmy mice that are very similar to today's Southern Pygmy Mouse.